Electroplating apparatus including means to disturb the boundary layer adjacent a moving electrode

ABSTRACT

Apparatus for transfer of mass, e.g. metallic ions to or from a relatively stationary body from or into a moving fluid. Means are included for continuously disturbing the fluid about the surface of the body to discourage the establishment of a steady state boundary layer of the fluid.

Feb. 6, 1973 United States Patent Anderson et al.

[56] References Cited UNITED STATES PATENTS ELECTROPLATING APPARATUSINCLUDING MEANS TO DISTURB THE BOUNDARY LAYER ADJACENT A MOVINGELECTRODE 204/2l2 ....204/l09 Fisher..................................204/2l2 2 536 912 1/1951 Corbett......................l.........3,477,926 11/1969 Snow et [75] Inventors: Ralph Anderson, Saratoga;Rodney 3560366 2/1971 B. Beyer, Sunnyvale, both of Calif.

[73] Assignee: Future Systems, Inc., Los Gatos, Primary Examiner johnHMack Calif.

Assistant ExaminerW. 1. Solomon Attorney-Thomas E. Schatzel ABSTRACT[22] Filed: Oct. 12, 1970 21 Appl. No.: 79,827

Apparatus for transfer of mass, e.g. metallic ions to or from arelatively stationary body from or into a mov- [52] 5355 ia/ ing fluid.Means are included for continuously disturblm Cl ing the fluid about thesurface of the body to discourage the establishment of a steady stateboundary layer of the fluid.

[58] Field of Search......204/l09, 212, 272, 289,281, 2047286 8 Claims,10 Drawing Figures PATENTEUFEB 61973 3.715.299 SHEET 10F 3 FIG. I

BY RODNEY B. BEYER ATTORNEY INVENTORS- RALPH ANDERSON PATENTEUFEB 6 I975SHEET 2 OF 3 DISTANCE BLT CATHODE I DISTANCE ANODE CATHODE ANODE BLTFIG.3

FIG.2

TIME F|G.4'

INVENTORS. RALPH ANDERSON ATTORNEY F lG.5

PATENTEDFEB 6 I975 3,715,299 SHEET 3 0F 3 9o FIG. l0 z INVENTORS.

RALPH ANDERSON RODNEY B. BEYER WWW ATTORNEY ELECTROPLATING APPARATUSINCLUDING MEANS TO DISTURB THE BOUNDARY LAYER ADJACENT A MOVINGELECTRODE BACKGROUND OF THE INVENTION The transfer of mass between aselective stationary body and a moving fluid is of prime concern innumerous chemical, electrochemical and heat transfer controlledprocesses. For example, in photographic l film development, the processis controlled by transfer of silver bearing species and other ions toand from the film surface. Also, in electrodeposition processes forextracting metal from a metal bearing solution to a collectingelectrode, the metal ion is transferred from the solution to thecollecting electrode. Electrodeposition structures heretofore availablecommonly include an anode, a cathode and means for circulating thesolution intermediate and past the cathode and anode. A voltagedifference is established across the electrodes such that the metal isattracted and deposited on the cathode and extracted from the solution.The collected metal may then be removed from the cathode. It has beenfound that the efficiency and success of transfer of the metal from thesolution to the collecting electrode is dependent on the boundary layerof fluid established about the relative stationary body of theelectrode.

There are presently various electrodeposition structures available.Commonly, as the metal content is depleted the impedance and the currentthrough the solution varies. Frequently, the potential value between theelectrodes is controlled as the concentration of metal within thesolution varies to effect certain separations. A common approach is toinstall a sensor cell to sense the metal concentration within thesolution. The potential across the electrodes is controlled responsiveto the sensor.

It has also been found that as the solution is transferred across theface of the collecting electrode, a boundary layer of relativelymotionless solution tends to form adjacent the cathode. The metal ionswithin this layer, commonly referred to as a boundary layer, aresubstantially depleted. The boundary layer, being relatively motionlessretains its position and impedes the transfer of metal from the solutionoutside the layer to the surface of the collecting electrode. Thus, toover come the boundary layer and reach the collecting electrode, themetal ions need be of sufficient velocity to penetrate said layer. Torealize the higher velocity, increased electrical potential need beestablished across the electrodes.

Electrodeposition apparatus is widely used in the recovery of silvermetal from silver bearing solutions such as that commonly used inphotographical fixing baths. The recovery of silver serves two keyfunctions to recover silver for further use and to regenerate the fixerfor further use. With prior art electrodeposition silver recoverysystems it has been found that as the laminar flow of the silver bearingfixer (hypo) solution takes place about an electrode, the electricalcurrent density must be controlled and varied as the silverconcentration varies in order to prevent breakdown of thiosulfate ionwhich causes silver sulfide precipitation. As silver is depleted fromthe solution, the excess electrical energy is more readily available foralternate reactions to occur. The insoluable silver sulfide precipitate,in turn, renders the solution unsuitable for further use. Thus, topreserve the collected silver and the solution for further use it isnecessary to avoid silver sulfiding. This requires control of theelectrical energy relative to the silver metal concentration of thesolution.

Besides the use of sensors and control of the current density, anotherapproach in the silver recovery art is to utilize a batch process. Anumber of recovery units are tied in cascade array and the silverbearing solution is permitted to flow from one recovery unit to another.The solution in each succeeding unit has less silver and each unit isoperated at less current density relative to the preceding unit. As thesolution flows from one unit to the next, the silver concentration isdiluted to the level of that particular unit and then transferred to thenext succeeding unit. It has been found that with the batch process, therecovery of silver becomes more time consuming in succeeding units asthe silver content is decreased and the current density decreased.Consequently, it becomes necessary to sacrifice either time or thedegree of extraction of silver from the solution in the terminal unit.

The prior art includes numerous various approaches to recover silverfrom solution and to rejuvenate hypo solution. Electrodepositionapparatus and methods are described in US. Pat. application Ser. No.851,697, filed Aug. 20, 1969, and now abandoned entitled Apparatus forSilver Recovery from Silver Bearing Solutions by Lawrence R. Francom andassigned to the assignee of the present invention. The disclosedstructure teaches an impeller to aid in circulating the solution aboutthe surface of the electrodes. This structure has proven tosubstantially minimize difficulties with sulfiding over that ofstructures theretofore available. US. Pat. No. 2,791,555 disclosesapparatus for recovery of silver from hypo solutions utilizing aplurality of disc shaped cathodes laterally spaced from a plurality ofanodes. The cathodes are continuously rotated within the solution.Canadian Patent No. 491,453 describes apparatus for recovery of silverfrom hypo solution utilizing a stationary cylindrical cathode and aplurality of carbon anodes driven in a circular path coaxial with thecathode and radially spaced from the interior and exterior surfaces ofthe cathode.

A tutorial review of silver recovery methods is provided in the paper byM. L. Schrieber, entitled Present Status of Silver Recovery inMotion-Picture Laboratories, published in the Journal of the SMPTE, June1965, Vol. 74, pp. 505-513. A tutorial paper by K. Hickman, C. Sanfordand W. Weyerts, entitled The Electrolytic Regeneration of Fixing Baths,published in the J.S.M.P.E., Oct, 1931, pp. 568-589, discussestheoretical and practical approaches to silver recovery and theconsiderations of sulfiding.

A. A. Rasch and J. I. Orabree authored a tutorial paper entitledElectrolytic Recovery of Silver From Fixing Baths at Low CurrentDensity, published in Photographic Science and Technique, Series II,Volume 2, Number 1, pp. 15-33, Feb. 1955. This paper recognizes that akey concern with electrodeposition apparatus is sulfiding. Nicholas J.Cedrone, in a paper entitled A Silver-Recovery Apparatus for Operationat High Current Density, published in the Journal of the S.M.P.T.E.,Volume 67, pp. 172-174, Mar. 1958, recognizes the concern of sulfidingand discusses an electrodeposition system utilizing external pumpagitation and high current density.

SUMMARY OF THE PRESENT INVENTION The present invention relates to animproved apparatus for mass transfer and control of boundary layer. Theinvention has proven to provide a significant improvement inelectrodeposition apparatus for recovery of metals from metal bearingsolutions. The invention teaches structure for facilitating the transferof solution about the face of the body moving relative to the solutione.g. a collecting electrode, and.

discouraging the creation of a relative motionless boundary layer ofsolution adjacent to said collecting electrode. This has been found inelectrodeposition embodiments to substantially decrease the tendency ofsulfiding even in the presence of high current densities.

In an exemplary embodiment of an electrodeposition apparatus including astationary electrode and a rotating electrode, the stationary electrodecarries a plurality of flexible tripper arms protruding towards the collecting surface of the collecting electrode and terminating adjacent tothe collecting electrode. Means are included to transfer the solutionintermediate the electrodes while the rotating electrode provides radialmotion to the solution. The tripper arms disturb and agitate thesolution adjacent the collecting electrode thereby discouraging buildupof a boundary layer.

The exemplary embodiment may be further adapted such that the inputportal of the apparatus for the solution is at a higher level than theoutput portal. Thus, the solution flows from the input portal to theoutput portal by fluid displacement in an axial direction about at leastone electrode while the rotating electrode simultaneously imparts radialmotion. The solution then exits at the lower level such that minimalmixing of the incoming solution with rejuvenated solution takes place.

BRIEF DESCRIPTION OF THE DRAWINGS boundary layer about the cathode ofthe structure of FIG. 1 with and without boundary layer trippers;

FIG. 5 is a sectional view along the line 55 of FIG. 1 and theoreticallyillustrating the eddy current action of the solution about the cathodeof FIG. 1 with boundary layer trippers;

FIG. 6 illustrates a partially sectioned plan view of an alternativeembodiment of an electrodeposition apparatus incorporating two anodesand the principles of the present invention;

FIG. 7 illustrates a sectional view taken along the line 7-7 of FIG. 6;and

FIG. 8-10 schematically illustrate other embodiments ofelectrodeposition apparatus incorporating the teachings of the presentinvention.

DESCRIPTION OF PREFERRED EMBODIMENTS The present invention has beenfound to be highly beneficial in apparatus for the recovery of silverfrom silver bearing solutions, such as fixative baths used in thephotographic industry. Such solutions are commonly referred to as hypoor fixer. In standard photographic film, silver is dispersed in agelatin material in the form of silver halides-primarily silver bromideand silver chloride. The remainder of the silver halides are believedreduced to pure silver due to exposure of light. During processing anddevelopment of the photographic film, the silver ion part of the silverhalides is extracted as a complex with a thiosulfate ion in the hyposolution. As the relative silver content is increased the hypo solutionmay, lose its effectiveness and utility for processing film.Simultaneously the silver within the hypo is a valuable commodity.Accordingly, the removal and collection of the silver from the hypo isan important concept to permit reuse of the hypo solution in processingfilm and recovery of the silver for reuse of the metal itself.

Various approaches have been taken to recover silver from hyposolutions. To date, electrolytic methods have been found to be overallthe most efficient, economical, and cleanest. A prime concern withelectrolytic systems heretofore available has been formation of silversulfide and other compounds during the recovery process. Sulfidingdecreases the purity of the collected silver and the effectiveness ofthe hypo for reuse. The formation of silver sulfide is a function ofcurrent density relative to the silver content within the solution. Asthe current relative to the silver content increases, the likelihood ofsulfiding increases. At the same time, however, as the current densitydecreases, the time required to extract silver from the solutionincreases and time efficiency decreases.

Approaches heretofore available have included incorporation of sensorsto sense the silver content of the hypo which hypo solution iscontinuously circulated through the electrodeposition apparatus. Anelectronic control network in turn responds to the sensor and controlsthe current within the solution between the electrodes. Thus, as thesilver content decreases, the current is decreased. Though said approachhas been found to improve the system efficiency and control ofsulfiding, further improvement is desireable for still more thoroughdepletion and efficient use of time. Prior art electrodeposition silverrecovery systems have proven that with hypo at low silverconcentrations, e.g. 0.20 troy ounces per gallon and less that maximumplating current densities without sulfiding are in the order of one totwo amperes per square foot of cathode plating surface area.

FIG. 1 illustrates a silver recovery electrodeposition apparatus,referred to by the general reference character 1 and incorporating theteachings of the present invention. The apparatus 1 includes a controlpanel-base support member 3 supporting about its top surface a retainerin the form of an enclosure 5 in which a volume of hypo or other silverbearing solution may be retained. The enclosure has a removable top 7and an integral bottom end wall 9. Preferably, the enclosure 5 includingthe walls 7 and 9 comprise a nonmetallic material compatible with thehypo solution. Such material may comprise a plastic, fiberglass,polyvinyl chloride, etc. Extending through the bottom surface 9 withinthe center of the enclosure 5 is a fixed cylindrical shaft housing 11.The housing 11 houses a cylindrical rotatable shaft 13. The shaft 13 iscomprised of an electrically conductive material and rotates about itsaxis within the housing 11 and the enclosure 5. The exterior of thehousing 11 carries a non-metallic coating compatible with the hypo, e.g.plastic, fiberglass, polyvinyl chloride, etc. The housing 11 is sealedabout the bottom surface 9 with an epoxy to prevent leakage. Within thehousing 11 about the shaft 13 is an axialthrust bearing (not shown) toprovide lateral and axial support and guidance to the shaft. About theupper end of the shaft 13 is an electrically conductive coupling member17 adapted to be secured to the shaft by means of a fastener 19 suchthat the coupling member 17 rotates responsive to the shaft 13. Atriangular shaped support member 21 comprised of a non-conductivematerial extends radially from the coupling member 17 and is secured tothe shaft 13. A plurality of three columns 23 are joined to the supportmember at the apexes and extend from said support member parallel to therotating shaft 13. A conducting strip 24 is secured to the top of theshaft 13 and extends laterally therefrom.

At the terminating end of the support columns 23 is supported animpeller 25 adapted to urge axial flow of the solution. The impeller 25is supported above the surface of the bottom wall 9. The impeller 25 isillustrated as carrying a plurality of openings 27 in the form ofarcurate slots of substantially equivalent size relative to one another.The impeller 25 carries an outer ridge 29 about the outside of thecolumn members 23. Engaging the exterior of the column members 23 andresting on the ridge 29 is a right circular cylinder cathode 31 referredto as the collecting electrode on which the silver is to be collected.The cathode 31 is substantially coaxial with the shaft 13. The cathode31, which is comprised of an electrically conductive material, e.g.stainless steel, is secured to the strip 24. Accordingly, application ofan electrical potential on the shaft 13 places the cathode 31 at asimilar potential and rotation of the shaft 13 places the cathode 31 insimultaneous rotation within the enclosure 5.

About the exterior surface of the cathode electrode and coaxialtherewith is a stationary right circular cylinder anode 33 preferablycomprised of a material having a high electrical conductivity and notsubject to attack by the hypo solution, e.g. stainless steel. The anode31 is substantially coaxial with the cathode 31 and is positioned flushagainst the interior side walls of the enclosure 5. The top of the anode33 is preferably below the level of the top of the cathode 31. The anode33 is desirably spaced laterally from the cathode 31 such that thecross-sectional area of the enclosed spacing between the anode 33 andthe exterior surface of the cathode 31 is substantially equal to thecross-sectional area intermediate the interior surface of the cathodeand shaft housing 11. This provides for substantially equal flow areasabout the interior and exterior surfaces of the cathode. Joined to theenclosure 5 about the top edge of the anode 33 is an outlet 34permitting exit of the solution from the enclosure 5. An inlet portal 35is included to permit entering of solution about the interior of thecathode 31.

In operation, a potential difference is established across the cathode31 and anode 33. Electrical current is forced through the hypo solutionby application of electrical voltage between the cathode 31 and theanode 33 and silver tends to be plated out in metallic form on thecathode surface. In a continuous flow process with the outlet portal 34open, entering incoming solution tends to force the solution axiallydownward relative to the interior of the cathode 31 and upward about thearea intermediate the cathode 31 and anode 33. The solution then flowsover the top edge of the anode 33 and out the outlet 34. The impeller 25tends to facilitate the flow of the solution. This method approximatesthe concept of a plug flow reactor.

The drive shaft 13 extends through the bottom wall 9 of the enclosure 5.About the exposed end, is a drive means including a gear 36 engaging adrive gear 37 driven by an electric drive motor 39 within the interiorof the support member 3. The panel-base support member 3 may be designedto include an ammeter 41 to indicate the current through the solutionintermediate the electrodes 31 and 33. Within the panelbase supportthere may be a suitable d.c. power source means (not shown) for applyinga d.c. voltage between the cathode 31 and anode 33. The panel-basemember 3 may also include an on-off switch 43, a voltage regulatorcontrol 45 extending to the power source means (not shown) and a speedregulator 47 to control the speed of the motor 39 and the rotationalvelocity of the shaft 13 and the cathode 31.

Accordingly, viewing the structure of FIG. 1, a hypo solution may beretained within the enclosure 5 and enter within the interior of thecathode 31. The solution may be contained if the portal 34 is closed orthere may be a continuous flow of solution if the portals 34 and 35 areopen. The motor 39 drives the cathode 31 in a rotary motion and the hyposolution circulates about the interior and exterior of the cathode. Asthe cathode 31 rotates hypo is continuously drawn down about theinterior of the cathode and upward between the cathode 31 and anode 33area. At the same time, the rotating cathode imparts radial motion tothe hypo. As the hypo is circulated, silver is plated on the surface ofthe cathode reducing the content of the silver ions within the hypo.After a substantial amount of silver is collected on the cathode 31, thecover 7 is removed, the strip 24 is disengaged and the entire unitarycathode may be removed by sliding it off of the column members 23. Afterremoval of the cathode 31, the collected silver may be removed byflexing the cathode. Also, the hypo is rejuvenated and may be reused inprocessing film.

The structure of FIG. 1 further carries a boundary layer tripper meansincluding a plurality of arms in the form of flexible flaps 49longitudinally engaged to and projecting from the exterior surface ofthe stationary anode 33. The flaps 49 are evenly radially spaced apartabout the anode. For example, in FIG. 1 the flaps 49 are spacedapproximately l20 apart. The flaps 49 extend longitudinallysubstantially end-to-end of the cathode 31 and terminate adjacent to theexterior collecting surface of the cathode 31 such that they extendsubstantially radially relative to said collecting surface. The flaps 49assume a concave shape and are comprised of an electricallynon-conductive material compatible with the hypo solution so as not tobe corroded or deteriorated to any substantial degree by the solution.To date, flaps 49 comprised of a saturated hydrocarbon elastomermaterial of approximately onesixteenth inch thickness have provencompatible. Such materials may include ethylene-propylene, rubber,neoprene rubber, n-butyl rubber, polyethylene, etc. The flaps areflexible and pivot about the point of engagement with the anode. Theflaps terminate adjacent to the cathode surface within the area of theboundary layer. For example, in structures similar to FIG. 1, the flaps49 terminate to leave a space in the order of onefourth to three-eighthsinch between the terminal end of the tabs and the cathode surface. Thedrive means moves the cathode 31 relative to the flaps 49 by driving thecathode 31 about the axis of the shaft 13.

The flaps 49 serve to disturb the hypo solution adjacent the surface ofthe cathode 31 and discourage establishment of a relative motionlesslayer of solution adjacent said cathode surface. The flaps 49 extendadjacent to the cathode within the area of this otherwise motionlesslayer. The theoretical explanation of the function and results of theboundary layer trippers is believed to be as follows. Theoretically, inmass transfer through liquids, a relative motionless layer of solutiontends to be formed adjacent to a relative stationary surface. The liquidoutside the layer, referred to as the bulk flow area, flows at a netbulk velocity. With the structure of FIG. 1 the net velocity Vr of theliquid within the bulk is dependent upon the vectorial sum of therotational velocity component and the axial velocity component. In theabsence of the boundary layer trippers 49, the velocity profile of thesolution between the anode 33 and the cathode 31 relative to the anodeis shown schematically by the solid line P in FIG. 2 assuming that theanode is stationary and the cathode is rotated at a velocity V,,,,. Thevelocity Vr of the fluid relative to the cathode 31 and the anode isschematically illustrated by the solid line V in FIG. 3 in the absenceof the boundary layer trippers. Viewing the diagram of FIG. 4, in theabsence of the boundary layer tripper flaps 49, as the viscous solutionis transferred axially about the cathode (indicated by the line 51 inFIG. 1) the flow fluid is retarded at and near the surface of thecathode 31 by the laminar boundary layer. The laminar boundary layer ofsolution tends to build up about the cathode surface and the depth d" ofthe layer is a function of the velocity of the fluid in the bulkrelative to the velocity of the relative stationary surface. Forexample, assuming the system is initially static and starting at a givenpoint, as the fluid is transferred axially across the relative axiallystationary surface, the thickness of the layer increases along thecathode surface as illustrated by the line Yo in FIG. 4. Analyzing theboundary layer as consisting of an incompressible fluid, the pressuredifferential of the fluid within the layer of zone (a) relative tooutside the layer of zone (b) is small (see FIG. 4), as the effect ofgravity is small. The force of the layer on the fluid attempting topenetrate the boundary layer may be sufficient to bring the fluid in theboundary layer to rest or cause flow in the reverse direction with theresult that a turbulent eddy current is set up, also shown in FIG. 4. Aregion of reverse flow then exists near the solid surface where theboundary layer has separated at the point P. The velocity of the fluidrises from zero at the surface of the solid boundary to a maximumnegative value (reverse flow) and falls again to zero along the lineP-Q'. Thus, P-Q' may be viewed as a zero velocity line. The velocitythen increases in the positive direction until it reaches the mainstream velocity Vr of the bulk at the edge of the boundary layer P0.

In the absence of the trippers 49, the line PQ represents thedevelopment of the boundary layer adjacent a given point on the cathodefrom start to steady state with the enclosure 5 containing a supply ofhypo. When steady state conditions are established a profile will be setup corresponding to a vertical section in FIG. 4. This is designated assection 8-8. The thickness d depends on the rotational velocity of thecathode or solid boundary. Somewhere, in the boundary layer, the flowwill separate at the point P and induce eddy formation and flowrecirculation. The induced eddy formation and flow recirculation is anormal result of high flow velocities in the absence of the trippers 49.As Vr increases, recirculation can increase the power required to movethe bulk fluid increases and there are practical limits as to powerinput.

Translation of a moving liquid and a stationary boundary condition (CaseI) to a moving solid boundary and a stationary or stagnant fluid (CaseII) are believed essentially equivalent in steady state flow conditions.However, in the pre-steady state conditions (unsteadystate boundarylayer build-up) they can be different. For example, in Case I with thefluid starting from rest, the magnitude of the boundary layer thicknessd decreases until steady state is reached while in Case II the magnitudeof d increases until steady state is reached. This is due primarily tothe fluid inertia. In both cases, however, as the velocity is increased,detachment of the fluid at the solid surface can occur which results ineddy formation and recirculation.

With the boundary layer trippers 49 attached to the anode 33, theboundary layer at the cathode 31 is continually disturbed at the cathodesurface. This is designated by the point P in FIG. 4 and with the movingsolid boundary cathode 31 and the stagnant bulk fluid, steady state isnever reached. Eddy currents are induced at point P adjacent to thesurface of the rotating cathode 31. These intense eddy currents persistfor a period of time and tend to dampen until the cathode has rotatedaround to another boundary layer tripper and the process is repeated.The induced eddy current phenomena adjacent to the surface of thecathode 31 thereby facilitates attraction to the cathode and transfer offluid to the immediate adjacent surface of the cathode.

In regard to electrodeposition, such as silver recovery or other masstransfer to or from a solid surface, the more prevalent the formation ofturbulent eddy currents, the faster the metal ions to be depleted orgenerated can be supplied or removed at the surface. Referring again toFIGS. 2 and 3,incorporation of the boundary layer tripper flaps 49 asdepicted in FIG. 1 cffectively reduces the radial flow passage betweenthe anode and cathode such that the profile and distribution arerepresented by the lines p and v respectively. The tripper flaps 49prevent the formation of laminar boundary layer about the cathode andintensifies eddy current flow at the cathode surface. The eddy currents,at a given point on the cathode tend to persist at the surface for asubstantial time period while the cathode rotates. The eddy currents atsaid given point tend to dampen intermediate successive trippersand arerejuvenated when the next tripper 49 is encountered. Said phenomena isillustrated schematically in FIG. illustrating a cross-sectional view ofthe structure of FIG. 1 taken along the line 5-5. As the cathoderotates, the boundary layer tripper flaps 49 tend to continuouslydisturb the solution'about the cathode surface. Simultaneously, thesupply of hypo is circulated adjacent to the cathode allowing readytransfer of silver ions from the circulating supply to the cathode.Accordingly, the electrical potential necessary to attract the silverions to reach the cathode is substantially decreased thereby decreasingthe necessary electrical power between the electrodes to realizeelectrodeposition. As the electrical potential is decreased the tendencyof silver sulfide at the cathode for a given current is deminished.

The boundary layer tripper flaps 49 have been found to provide improvedefficiency in electrodeposition in various ways. First, the boundarylayer is decreased thereby facilitating transfer of silver ions from thebulk solution to the cathode surface. Second, the solution is circulatedimmediately adjacent to the cathode surface thereby facilitatingtransfer of the silver ions from the circulating solution to thecathode. Third, the continuous circulation has been found to virtuallyeliminate the tendency of sulfiding thus permitting increased currentdensities, even as the silver ion content decreases. Embodiments ofsilver recovery units incorporating boundary layer tripper flaps 49 havebeen found to accommodate current densities in the order of 100 amperesper square foot of cathode surface area with hypo solutions of silverconcentration in the order 0.1 troy ounces per gallon without detectablesulfiding of the silver.

The incorporation of the boundary layer trippers have been found topermit units similar to the unit 1, to perform similar to a batchprocess without the reaction times necessary in batch processes. Withthe unit 1 as the solution transfers axially, the concentration ofsucceeding increments of solution of axial movement have less silverconcentration. At the same time, the high degree of agitation has beenfound to permit a uniform potential across the electrodes notwithstanding the changing concentrations along the axial plane.

FIG. 6 illustrates a plan view of a further embodiment of a silverrecovery unit incorporating the teachings of the present invention andreferred to by the general reference character 60. The unit 60,exteriorally is similar to the unit 1 of FIG. 1 such that elementscommon to those of the unit 1 carry the same reference with a primedesignation. The unit 60 is designed to incorporate a pair of stationaryright circular cylinder anodes 62 and 64. A rotatable right circularcylinder cathode 66 is positioned intermediate the anodes 62 and 64 suchthat each of the anodes is adjacent a collecting surface of the cathode66. The anode 62 carries a plurality of evenly spaced boundary layertripper flaps 49 and the anode 64 carries a plurality of evenly spacedboundary layer tripper flaps 49". The support member 21 is comprised ofan electrically conductive material such that the cathode 66 is at thesame electrical potential as the shaft 13. FIG. 7 illustrates across-sectional segment of the structure 60 taken along the line 7-7.Each of the anodes 62 and 64 carry a positive potential relative to thecathode 66 such that silver is plated on both exterior surfaces of thecathode 66 as illustrated by the layers of collected silver 68 and 70.In the embodiment 60 the boundary layer tripper flaps 49 and 49simultaneously disturb the solution adjacent both exterior surfaces ofthe cathode to facilitate plating on both of said surfaces as thecathode is rotationally driven about the shaft 13. The eddy currents areintensified at both surfaces of the cathode. The plating on bothsurfaces of the cathode is desired to be uniform. In units heretoforeavailable, the spacing between the cathode and the anodes was criticalfor uniform plating. The boundary layer trippers 49 and 49 aid inuniform plating because the electrical resistance at the surface of thecathode is decreased, due to decrease or elimination of the boundarylayer. Consequently, the spacing between the cathode and anodes has beenfound not to be as critical as heretofore known.

FIG. 8 schematically illustrates a plan view of a further embodiment ofan electrodeposition apparatus incorporating the teachings of thepresent invention and referred to by the general reference character 72.The apparatus 72 includes a stationary right cylindrical anode 74 and arotating right cylindrical cathode 75. Suspended intermediate thereofand adjacent the cathode are a plurality of arms in the form ofstationary flexible tubes or rods 76. The rods 76 are positionedimmediately adjacent to the surface of the cathode and function as theboundary layer trippers. There may also be included impeding vanes 78 toimpede rotational flow of the solution and to agitate the solution. Asthe cathode rotates, the tubes or rods 76, which are stationary, createeddy currents adjacent to the cathode to discourage the establishment ofa boundary layer.

FIG. 9 schematically illustrates a further embodiment of anelectrodeposition apparatus incorporating the teachings of the presentinvention and referred to by the general reference character 82. Theapparatus 82 includes a stationary right cylindrical anode 84 and astationary right cylindrical cathode 85. A rotating member 87 supports aplurality of arms in the form of flexible, concave boundary layertrippers 88 similar to the flaps 49. The flaps 88 are rotated coaxiallyabout the area intermediate the anode 84 and cathode 85. The flaps 88terminate immediately adjacent to the surface of the cathode andfunctions to discourage the establishment of a boundary layer about thecathode 85.

A further embodiment of electrodeposition apparatus incorporating theteachings of the present invention and referred to by the generalreference character. 90 is illustrated in FIG. 10. The apparatus 90includes a drive shaft 92 supporting a plurality of disc cathodes 94. Aplurality of arms in the form of flexible members 96 protrude to withinthe area intermediate adjacent the discs 94. The members 96 serve asboundary layer trippers to disturb the solution about said discs anddiscourage the establishment of boundary layers about the disc surfaces.The members 96 are supported by a column 98. An anode 100 is included.The cathodes 94 are moved relative to the boundary layer trippers 96 bymaintaining the trippers 96 stationary while the cathodes are rotated bythe shaft 92.

We claim:

1. Apparatus for the electrodeposition of a metal from a body ofsolution comprising:

a retainer for retaining a volume of a solution bearing a metal to berecovered;

a first electrode member within the retainer and having a firstcollecting surface to receive and collect said metals;

a second electrode member within the retainer and disposed in spacedapart relationship with said first electrode member, the space betweensaid first collecting surface and second electrode defining a chamberfor passage of a body of said solution;

means for establishing an electrical potential difference between saidfirst electrode member and said second electrode member;

means for moving said first electrode member in a first direction, suchmovement tending to establish proximate said first collecting surface alaminar boundary layer of fluid having a substantial component of motionin said first direction responsive to movement of said first electrode;and

boundary layer tripper means disposed within said chamber and stationaryrelative to said second electrode member, said boundary layer trippermeans extending into said boundary layer of fluid for creating eddycurrents of fluid within said boundary layer as said first electrodemember is moved.

2. The apparatus of claim 1 wherein said first and second electrodes arein the form of a right circular cylindrical body disposed in coaxialrelationship, and wherein said boundary layer tripper means includes atleast one elongated member having a longitudinal dimension extendingsubstantially parallel with said first collecting surface.

3. The apparatus of claim 1 in which the boundary layer tripper meansincludes a plurality of arms positioned within said chamber, each ofsaid arms having a longitudinally extending end, each of said armspositioned with its respective longitudinal dimension extendingsubstantially parallel with said first collecting surface.

4. The apparatus of claim 3 in which said arms are in the form of aplurality of flaps each comprised of a flexible material compatible withthe solution to be retained within the retainer.

5. Apparatus for electrodeposition of a metal from a solutioncomprising, in combination:

a retainer for retaining a volume of solution bearing a metal to berecovered;

a first electrode member within the retainer and having a firstcollecting surface and a second collecting surface, said first andsecond collecting surfaces being positioned to be exposed to thesolution within the retainer to receive and collect said metal;

a second electrode member within the retainer and spaced from the firstcollecting surface of the first electrode, the first and secondelectrodes positioned relative to one another to permit said solution tocirculate intermediate the first and second electrodes;

a third electrode member within the retainer and spaced from the secondcollecting surface of the first electrode with the first electrodepositioned intermediate the second and third electrodes, the first andthird electrodes positioned relative to one another to permit saidsolution to circulate intermediate the first and third electrodes;

electrical power means for establishing an electrical potential on thefirst and second electrodes relative to one another and on the first andthird electrodes relative to one another;

boundary layer tripper means intermediate the first and second electrodemembers and intermediate the first and third electrode members fordisturbing the solution adjacent the first and second collectingsurfaces, the boundary layer tripper means including at least one armpositioned within the retainer intermediate the first and secondelectrodes and projecting toward said first collecting surface and atleast one arm positioned within the retainer intermediate the first andthird electrodes and projecting toward said second collecting surface;and

drive means for providing relative motion between the boundary layertripper means and said first and second collecting surfaces.

6. The apparatus of claim 5 in which said arms are comprised of aflexible non-conductive material compatible with the solution, said armsterminating adjacent to the respective first and second collectingsurfaces.

7. The apparatus of claim 6 in which the boundary layer tripper meansincludes a plurality of said arms with the second and third electrodeseach being engaged to a plurality of arms, the arms engaged to thesecond electrode being substantially uniformally spaced relative to oneanother about the first collecting surface of the first electrode, andthe arms engaged to the third electrode being substantially uniformallyspaced relative to one another about the second collecting surface ofthe first electrode.

8. The apparatus of claim 5 in which the first, second and thirdelectrode members are in the form of right circular cylinders coaxialabout a common axis with the first electrode member positionedintermediate the second and third electrodes, the first electrode memberbeing engaged to the drive means, the drive means being adapted torotate the first electrode about said common axis;

the boundary layer tripper means includes a first set of boundary layertrippers and a second set of boundary layer trippers, said first set ofboundary layer trippers being positioned intermediate the first and thesecond electrode members and adjacent said first collecting surface fordisturbing the solution adjacent to said first collecting surface, andsaid second set of boundary layer trippers being positioned intermediatethe first and the third electrode members and adjacent said secondcollecting surface for disturbing the solution adjacent to said secondcollecting surface, each of the first set and each of the second set ofboundary layer tripper means extending substantially normal to saidfirst and second collecting surfaces of the first electrode;

an inlet portal means to permit a solution to enter the retainerintermediate the first and third electrodes about an axial terminal endof the second collectl0

1. Apparatus for the electrodeposition of a metal from a body ofsolution comprising: a retainer for retaining a volume of a solutionbearing a metal to be recovered; a first electrode member within theretainer and having a first collecting surface to receive and collectsaid metals; a second electrode member within the retainer and disPosedin spaced apart relationship with said first electrode member, the spacebetween said first collecting surface and second electrode defining achamber for passage of a body of said solution; means for establishingan electrical potential difference between said first electrode memberand said second electrode member; means for moving said first electrodemember in a first direction, such movement tending to establishproximate said first collecting surface a laminar boundary layer offluid having a substantial component of motion in said first directionresponsive to movement of said first electrode; and boundary layertripper means disposed within said chamber and stationary relative tosaid second electrode member, said boundary layer tripper meansextending into said boundary layer of fluid for creating eddy currentsof fluid within said boundary layer as said first electrode member ismoved.
 2. The apparatus of claim 1 wherein said first and secondelectrodes are in the form of a right circular cylindrical body disposedin coaxial relationship, and wherein said boundary layer tripper meansincludes at least one elongated member having a longitudinal dimensionextending substantially parallel with said first collecting surface. 3.The apparatus of claim 1 in which the boundary layer tripper meansincludes a plurality of arms positioned within said chamber, each ofsaid arms having a longitudinally extending end, each of said armspositioned with its respective longitudinal dimension extendingsubstantially parallel with said first collecting surface.
 4. Theapparatus of claim 3 in which said arms are in the form of a pluralityof flaps each comprised of a flexible material compatible with thesolution to be retained within the retainer.
 5. Apparatus forelectrodeposition of a metal from a solution comprising, in combination:a retainer for retaining a volume of solution bearing a metal to berecovered; a first electrode member within the retainer and having afirst collecting surface and a second collecting surface, said first andsecond collecting surfaces being positioned to be exposed to thesolution within the retainer to receive and collect said metal; a secondelectrode member within the retainer and spaced from the firstcollecting surface of the first electrode, the first and secondelectrodes positioned relative to one another to permit said solution tocirculate intermediate the first and second electrodes; a thirdelectrode member within the retainer and spaced from the secondcollecting surface of the first electrode with the first electrodepositioned intermediate the second and third electrodes, the first andthird electrodes positioned relative to one another to permit saidsolution to circulate intermediate the first and third electrodes;electrical power means for establishing an electrical potential on thefirst and second electrodes relative to one another and on the first andthird electrodes relative to one another; boundary layer tripper meansintermediate the first and second electrode members and intermediate thefirst and third electrode members for disturbing the solution adjacentthe first and second collecting surfaces, the boundary layer trippermeans including at least one arm positioned within the retainerintermediate the first and second electrodes and projecting toward saidfirst collecting surface and at least one arm positioned within theretainer intermediate the first and third electrodes and projectingtoward said second collecting surface; and drive means for providingrelative motion between the boundary layer tripper means and said firstand second collecting surfaces.
 6. The apparatus of claim 5 in whichsaid arms are comprised of a flexible non-conductive material compatiblewith the solution, said arms terminating adjacent to the respectivefirst and second collecting surfaces.
 7. The apparatus of claim 6 inwhich the boundary layer trIpper means includes a plurality of said armswith the second and third electrodes each being engaged to a pluralityof arms, the arms engaged to the second electrode being substantiallyuniformally spaced relative to one another about the first collectingsurface of the first electrode, and the arms engaged to the thirdelectrode being substantially uniformally spaced relative to one anotherabout the second collecting surface of the first electrode.